Model of the Atom

\({}^{A}_{Z}X\), where \(A=\) the nucleon number, and \(Z=\) the proton number

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Isotopes will have a different number of neutrons

The nucleon expressed as a mass in grams approximates to the mass of one mole of a given isotope.
One mole of a substance contains the same number of entities as there are atoms in 12g of carbon-12; \(6.02\times 10^{23}\).